The invention relates generally to a transport system, and particularly to a transport system capable of transporting articles in a fabrication system.
In a conventional fab, wafer processing steps are performed in discrete areas (bays). Wafers are transported between bays (interbay) and within bays (intrabay). FIG. 1 is a schematic view showing a conventional fabrication system layout. A fabrication system 100 capable of fabricating semiconductor wafers comprises tool bays 11 and 15, and a plurality of transport systems. Each tool bay comprises, in general, a number of tools for performing wafer fabrication functions. For example, tool bays 11 and 15 comprise tools 11a to 11h, and tools 15a to 15h, respectively. The transport system uses vehicles for storing and transporting wafers. Within the fabrication system 100, an interbay transport system 19 transfers vehicles between tool bays; and intrabay transport systems 12 and 16 transport vehicles within tool bays 11 and 15, respectively. Wafers cannot be transported between interbay transport systems and intrabay transport systems directly, but are transferred through stockers located at the end of the bays. As shown in FIG. 1, stockers 13 and 17 serve tool bays 11 and 15, respectively. Stocker 13 has interbay load ports 139a, 139b and intrabay load ports 131a, 131b for transporting wafers to and from interbay transport system 19 and intrabay transport system 12, respectively. Stocker 17 has interbay load ports 179a, 179b and intrabay load ports 171a, 171b for transporting wafers to and from interbay transport system 19 and intrabay transport system 16, respectively.
Typically, an intrabay overhead transport (OHT) vehicle delivers wafers directly to the tool. Vehicles loaded with wafers are picked from a process load port by an intrabay OHT vehicle and delivered to the local stocker. A stocker robot then moves the wafers to the interbay interface to be loaded onto an overhead shuttle (OHS) interbay transport system. An OHS vehicle then moves the wafers to the stocker closest to the next process step for storage. When the wafers are to be processed, an available intrabay OHT vehicle goes to the local stocker to retrieve the wafers, transports the wafers inside the bay to the tool location, and “places” the wafers onto the tool loadport. This highly mechanized and segregated transport system requires two stocker transfers to complete the move job, OHT vehicles consigned to their respective intrabay loop, and various control systems for each participating member (intrabay, stocker, interbay).
Recently, OHT devices have been added to the interbay system. In this situation, vehicles are not confined in a particular bay, and the number of vehicles in a particular bay is generally managed manually. Additionally, according to a conventional method, the OHT is used in a tool-to-tool transport, even though it is more efficient using the OHS system. Thus an efficient traffic management cannot be achieved by conventional methods.